FAIR VALUE OF FINANCIAL INSTRUMENTS (Details) - USD ($) $ in Millions	Jun. 30, 2018	Sep. 30, 2017	Jun. 30, 2017	Sep. 30, 2016
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	$ 6.9	$ 7.4	$ 8.3	$ 5.2
Spire Missouri
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	3.5	2.5	2.6	2.1
Spire Alabama
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	0.0	0.1	0.1	$ 0.0
Quoted Prices in Active Markets (Level 1)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	6.9	7.4	8.3
Short-term debt	0.0	0.0	0.0
Long-term debt, including current portion	0.0	0.0	0.0
Quoted Prices in Active Markets (Level 1) | Spire Missouri
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	3.5	2.5	2.6
Short-term debt	0.0	0.0	0.0
Long-term debt, including current portion	0.0	0.0	0.0
Quoted Prices in Active Markets (Level 1) | Spire Alabama
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents		0.1	0.1
Short-term debt	0.0	0.0	0.0
Long-term debt, including current portion	0.0	0.0	0.0
Significant Observable Inputs (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	0.0	0.0	0.0
Short-term debt	191.0	477.3	450.7
Long-term debt, including current portion	2,173.7	2,210.3	2,033.0
Significant Observable Inputs (Level 2) | Spire Missouri
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	0.0	0.0	0.0
Short-term debt	128.6	203.0	260.2
Long-term debt, including current portion	1,005.0	1,056.9	883.9
Significant Observable Inputs (Level 2) | Spire Alabama
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents		0.0	0.0
Short-term debt	69.6	169.9	114.9
Long-term debt, including current portion	317.0	269.4	267.6
Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	6.9	7.4	8.3
Short-term debt	191.0	477.3	450.7
Long-term debt, including current portion	2,180.0	2,095.0	1,925.3
Carrying Amount | Spire Missouri
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	3.5	2.5	2.6
Short-term debt	128.6	203.0	260.2
Long-term debt, including current portion	974.2	973.9	804.5
Carrying Amount | Spire Alabama
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents		0.1	0.1
Short-term debt	69.6	169.9	114.9
Long-term debt, including current portion	322.5	247.8	247.7
Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	6.9	7.4	8.3
Short-term debt	191.0	477.3	450.7
Long-term debt, including current portion	2,173.7	2,210.3	2,033.0
Fair Value | Spire Missouri
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	3.5	2.5	2.6
Short-term debt	128.6	203.0	260.2
Long-term debt, including current portion	1,005.0	1,056.9	883.9
Fair Value | Spire Alabama
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents		0.1	0.1
Short-term debt	69.6	169.9	114.9
Long-term debt, including current portion	$ 317.0	$ 269.4	$ 267.6